check out Greg V's excellent demo of the 18 and the MI-6 as he plays the same riffs through both amps and you can hear the differences well. There is a very nice tele clip on the Reinhardt site of the MI-6. I personally like the cleans on the MI-6 better as they seem fuller and more balanced. The 18 is definitely the way to go if you want classic Marshall crunch and I also like the cleans on the 18. The MI-6 has a thicker low mid kind of thing on the overdrive and it sounds great...both great amps...I thought I would sell the 18 once I got the MI-6 but they are both still here as they seem to have different virtues for me. I tend to play the MI-6 clean while I love the crunchy tones on the 18.
hope that helps...call Bob Reinhardt and he will definitely help you out as he is a really nice guy who really stands behind his products. great build quality and Bob's wife Sue does the tolex on the amps and it is flawless.